Hey I wanna challenge myself and make something that allows be to monitor my laps in the pool via a computer. I was think of something like a waterproof pressure mat hanging in the water, connected to a USB or Serial port connected to the computer.
You want to use a mains powered appliance in the vacinity of a swimming pool? ensure that the computer is correctly earthed and/or use an isolating transformer or RCD ....
maybe replace one of the buttons of an old mouse with wires leading to the pressure pad.
from the safety standpoint and to keep the PC away from corrosive chemicals it may be better tu use an air-filled pressure pad connected by a long hose to a remotely located pressure switch connected to a old mouse (or directly to pins used for the fire button on the joystick port etc...).
suitable pressure switches can be salvaged from many models of washing machines (used in the water level detector)
as for the pressure pad, maybe a wheelbarrow inner-tube between two sheets of stiff plastic (acrylic/lexan/fibreglass) etc
(the pressure switch could probably be made this way too)
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